Fuel cells with hydrophobic diffusion medium

ABSTRACT

Diffusion media for use in PEM fuel cells are provided with silicone coatings. The media are made of a porous electroconductive substrate, a first hydrophobic fluorocarbon polymer coating adhered to the substrate, and a second coating comprising a hydrophobic silicone polymer adhered to the substrate. The substrate is preferably a carbon fiber paper, the hydrophobic fluorocarbon polymer is PTFE or similar polymer, and the silicone is moisture curable.

FIELD OF THE INVENTION

This invention relates to fuel cells with hydrophobic diffusion medium.In particular, the invention relates to fuel cell diffusion media havinghydrophobic silicone coatings.

BACKGROUND OF THE INVENTION

Fuel cells are increasingly being used as a power source for electricvehicles and other applications. An exemplary fuel cell has a membraneelectrode assembly (MEA) with catalytic electrodes and a proton exchangemembrane (PEM) formed between the electrodes. Gas diffusion media playan important role in PEM fuel cells. Generally disposed betweencatalytic electrodes and flow field channels in the fuel cell, theyprovide reactant and product permeability, electronic conductivity, andheat conductivity, as well as mechanical strength needed for properfunctioning of the fuel cell.

During operation of the fuel cell, water is generated at the cathodebased on electrochemical reactions involving hydrogen and oxygenoccurring within the MEA. Efficient operation of a fuel cell depends onthe ability to provide effective water management in the system. Forexample, the diffusion media prevent the electrodes from flooding (i.e.,filling with water and severely restricting O₂ access) by removingproduct water away from the catalyst layer while maintaining reactantgas flow from the bipolar plate through to the catalyst layer.

The gas diffusion media are generally constructed of carbon fibercontaining materials. Although carbon fibers are themselves relativelyhydrophobic, it is usually desirable to increase the hydrophobicity orto at least treat the carbon fiber with a more stable hydrophobiccoating. Adding a hydrophobic agent such as polytetrafluoroethylene(FTFE) to the carbon fiber diffusion media is a common process forincreasing the hydrophobicity. This process is normally done by dippingcarbon fiber papers into a solution that contains PTFE particles andother wetting agents, such as non-ionic surfactants.

Fuel cell stacks can contain a large number of fuel cells depending onthe power requirement of the application. For example, typical fuelstacks have up to 200 individual fuel cells and more. Because the fuelcells in the stacks operate in series, a weakness or poor performance inone cell can translate into poor performance of the entire stack. Forthis reason, it is desirable for every fuel cell in the stack to operateat high efficiency.

Because typical fuel stacks contain so many individual fuel cells, ithas been observed that, even with a high degree of reliability ofmanufacture of diffusion media, it is sometimes observed that anindividual or several diffusion media will have less than optimumperformance, especially at a high relative humidity. When that occurs, afuel stack containing such a fuel cell will generally exhibit less thanoptimum performance. Thus, diffusion media with enhanced hydrophobicproperties and methods for producing them that lead to consistentresults among hundreds of fuel cells in a single fuel stack would be anadvance in the art.

SUMMARY OF THE INVENTION

In one aspect of the invention, silicone coatings are provided ondiffusion media for use in fuel cells, such as PEM fuel cells. Thediffusion media are made of a porous conductive substrate, a firsthydrophobic fluorocarbon polymer coating adhered to the substrate, and asecond coating comprising a hydrophobic silicone polymer adhered to thesubstrate. In various embodiments, the porous conductive substrate is acarbon fiber paper or other conductive substrate commonly used in a PEMfuel cell, and the hydrophobic fluorocarbon polymer is a hydrophobicpolymer such as polytetrafluoroethylene (PTFE).

In various embodiments, the second coating is applied to a conductivesubstrate on which the first coating has already been applied; the firstcoating adheres to a substrate over a major part of the surface area ofthe substrate, and the second coating (the silicone polymer) adheres toan area or areas of the substrate that are not completely covered by thefirst coating. The second coating is preferably applied by contactingthe substrate containing the first coating with a silicone composition.Preferably, the silicone composition contains components that cure toform the hydrophobic silicone polymer adhering to the substrate. In apreferred embodiment, the silicone polymer system is curable by theaction of moisture and typically at room temperature.

Performance of PEM fuel stacks containing up to 200 or more individualfuel cells containing such diffusion media is improved and found to bemore reliable, by virtue of the improved hydrophobic nature of theindividual diffusion media in the stack. Accordingly, methods areprovided for making the diffusion media and for improving theperformance of fuel cell stacks containing individual fuel cellscontaining the media. The methods involve contacting a conductive poroussubstrate on which a hydrophobic fluorocarbon polymer is adhering with asilicone composition comprising, in a preferred embodiment, a moisturecurable silicone resin.

In various embodiments, fuel cell stacks are assembled wherein each fuelindividual fuel cell of the stack contains a diffusion medium coatedwith a hydrophobic silicone as described. In some embodiments, operationof a fuel cell stack is improved by first identifying any individualfuel cell that is performing poorly by virtue of having a diffusionmedium that is too hydrophilic, and treating that diffusion medium ofthat fuel cell with the silicone composition as described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell stack

FIG. 2 shows current voltage curves of treated diffusion media.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In one embodiment, a diffusion medium suitable for use in a PEM fuelcell is made of a porous conductive (i.e., electroconductive) substratehaving a first coating comprising a hydrophobic fluorocarbon polymeradhered to substrate. A second coating comprising a hydrophobic siliconepolymer is also adhered to the substrate. In various embodiments, thesubstrate comprises a carbon fiber based diffusion medium, such as acarbon fiber paper. In typical embodiments, the hydrophobic fluorocarbonpolymer is a hydrophobic material such as polytetrafluoroethylene(PTFE).

In another embodiment, a method for making a fuel cell diffusion mediumfor use in a PEM fuel cell comprises first coating a porous conductivesubstrate with a hydrophobic fluorocarbon polymer. After the substrateis coated with the hydrophobic fluorocarbon polymer, the fluorocarbonpolymer coated substrate is then contacted with a silicone compositioncomprising a moisture curable silicone resin. In various embodiments,the hydrophobic fluorocarbon polymer is applied to a loading of 1 to 20%by weight based on the total weight of the substrate, and the siliconeis applied from about 0.01 to about 5%, preferably from about 0.1 to 2%by weight, based on the total weight of the coated substrate. The methodcan further be applied to diffusion media that contain a microporouslayer coating on one side of the conductive substrate. The microporouslayer contains a fluorocarbon polymer and conductive particles, andgenerally has pore sizes much smaller than the pore sizes on the side ofthe substrate not coated with the microporous layer.

In another embodiment, fuel cells are provided that contain hydrophobicdiffusion media as described herein. In a further embodiment, fuel cellstacks are provided that contains a plurality of the fuel cells.

In another embodiment, a method is provided for improving the highhumidity performance of a PEM fuel cell stack. The stack contains aplurality of PEM fuel cells, each of the fuel cells containing acathode, an anode, and a polyelectrolyte membrane disposed between thecathode and the anode, and further containing flow fields adjacent theelectrodes (i.e., the anode and cathode). A fluoropolymer-coateddiffusion medium is disposed between at least one of the electrodes andits flow field, that is, at least one of cathode and the cathode flowfield and the anode and the anode flow field. That is to say, theindividual fuel cells contain a diffusion medium on the cathode sideand/or the anode side. The method involves contacting the fluoropolymercoated diffusion medium with a silicone composition that contains amoisture curable silicone resin. In various embodiments, the methodresults in the application of a hydrophobic silicone coating on areas ofthe fluorocarbon polymer coated diffusion medium that for one reason oranother, including random or unpredicted variations in fluorocarboncoating processes, contain areas not completely coated with fluorocarbonpolymer, which areas are therefore more hydrophilic than the rest of thediffusion medium.

In various embodiments, the method is carried out by operating a fuelcell stack and identifying any individual cells in the stack that arenot performing as expected. In some embodiments, the fuel cellperforming poorly is removed from the stack and the diffusion mediumtreated as described with a silicone coating. Thereafter the fuel cellstack is reassembled.

Fuel cell stacks contain a plurality of fuel cells, the number ofindividual cells depending on the power and voltage requirements of theapplication. In automotive use, typical fuel cell stacks contain 50 ormore individual fuel cells and can contain up to 400, 500, or even more.Power requirements in various applications can also be met by providinga number of modules comprising individual fuel cell stacks. The modulesare designed to work in a series to provide adequate power and are sizedto fit within the available packaging.

FIG. 1 is an expanded view showing some details of the construction of atypical multi-cell stack, showing just two cells for clarity. Thebipolar fuel cell stack 2 has a pair of membrane electrode assemblies(MEA) 4 and 6 separated from each other by an electrically conductivefuel distribution element 8, hereinafter bipolar plate 8. The MEA's 4and 6 and bipolar plate 8 are stacked together between stainless steelclamping plates or end plates 10 and 12 and end contact elements 14 and16. The end contact elements 14 and 16, as well as both working faces ofthe bipolar plate 8, contain a flow field of a plurality of grooves orchannels 18, 20, 22, and 24 respectively, for distributing fuel andoxidant gases (i.e. hydrogen and oxygen) to the MEA's 4 and 6.Non-conductive gaskets 26, 28, 30, and 32 provide seals and electricalinsulation between several components of the fuel cell stack. Gaspermeable conductive materials used as gas diffusion media are typicallycarbon/graphite diffusion papers 34, 36, 38, and 40 that press upagainst the electrode faces of the MEA's 4 and 6. The end contactelements 14 and 16 press up against the carbon graphite diffusion media34 and 40 respectively, while the bipolar plate 8 presses up against thediffusion medium 36 on the anode face of MEA 4, and against carbongraphite diffusion medium 38 on the cathode face of MEA 6. Oxygen issupplied to the cathode side of the fuel cell stack from storage tank 46by appropriate supply plumbing 42, while hydrogen is supplied to theanode side of the fuel cell from storage tank 48, by appropriate supplyplumbing 44. Alternatively, ambient air may be supplied to the cathodeside as an oxygen source and hydrogen may be supplied to the anode froma methanol or gasoline reformer. Exhaust plumbing (not shown) for boththe hydrogen and oxygen sides of the MEA's 4 and 6 will also beprovided. Additional plumbing 50, 52, and 54 is provided for supplyingliquid coolant to the bipolar plate 8 and end plates 14 and 16.Appropriate plumbing for exhausting coolant from the coolant bipolarplate 8 and end plate 14 and 16 is also provided, but not shown.

Individual fuel cells contain a proton exchange membrane disposedbetween electrodes. The electrodes are an anode and a cathode for use incarrying out the overall production of water from fuel containinghydrogen and an oxidant gas containing oxygen. In various embodiments,the electrodes contain carbon support particles on which smallercatalyst particles (such as platinum) are disposed, the carbon andcatalyst supported generally on a porous and electroconductive materialsuch as carbon cloth or carbon paper. Suitable electrodes arecommercially available; in some embodiments, the anode and cathode aremade up of the same material.

The electrically conductive porous material or substrate for use as thediffusion media in the invention is in general a porous planar flexiblematerial that may be wetted by water or other solvents associated withsolutions of polymers as described below. In various embodiments, theporous material (also called a sheet material) is made of a woven ornon-woven fabric or paper.

In a preferred embodiment, the sheet material is made of a carbon fibersubstrate such as carbon fiber paper. Carbon fiber-based papers may bemade by a process beginning with a continuous filament fiber of asuitable organic polymer. After a period of stabilization, thecontinuous filament is carbonized at a temperature of about 1200°C.-1350° C. The continuous filaments can be woven into carbon cloth orchopped to provide shorter staple carbon fibers for making carbon fiberpaper. These chopped carbon fibers are made into carbon fiber papersheets or continuous rolls through various paper making processes.Thereafter, in an illustrative embodiment, the carbon fiber papers areimpregnated with an organic resin and molded into sheets or rolls. Thewoven carbon cloth and the molded carbon paper sheets or rolls are thencarbonized or graphitized at temperatures above 1700° C. Suitable carbonfiber-based substrates are described, for example in Chapter 46 ofVolume 3 of Fuel Cell Technology and Applications, John Wiley & Sons,(2003), the disclosure of which is helpful for background and isincorporated by reference. In various embodiments, the substrates takethe form of carbon fiber paper, wet laid filled paper, carbon cloth, anddry laid filled paper.

Carbon fiber papers may be thought of as a non-woven fabric made ofcarbon fibers. Carbon fiber paper is commercially available in a varietyof forms. In various embodiments, for example, the density of the paperis from about 0.3 to 0.8 g/cm³ or from about 0.4 to 0.6 g/cm³, and thethickness of the paper is from about 100 μm to about 1000 μm, preferablyfrom about 100 μm to about 500 μm. Typical porosities of commerciallyavailable papers are from about 60% to about 80%. Suitable carbon fiberpapers for use in fuel cell applications as described herein areavailable for example from Toray Industries USA. An example ofcommercially available carbon fiber paper from Toray is TGPH-060, whichhas a bulk density of 0.45 gm/cm³ and is approximately 180 microns(micrometers) thick.

In one aspect, the hydrophobic fluorocarbon polymer is one that willsettle out of an emulsion or precipitate out of a solution under theevaporating conditions described below. Preferably, the polymerdeposited onto the sheet material is one that will remain stably incontact with the portions of the sheet during conditions of its use inthe eventual end application, such as a diffusion medium in a fuel cell.The compatibility or stability of the polymer in contact with thesubstrate may be enhanced by certain post-curing steps where the coatedsheet material is heated to a high temperature (e.g., 380° C. for PTFE)to fix the structure of the polymer on the sheet material.

The fluorocarbon polymer generally imparts a hydrophobic character tothe substrate sheet material where the polymer is deposited. Byconvention, the polymer is considered to render the surface of thesubstrate hydrophobic if the surface free energy of the polymer materialis less than the surface free energy of the sheet material itself.Surface free energy of the polymer and the sheet material may bemeasured by and correlated to the contact angle of water in contact withthe polymer or sheet material, respectively. For example, if the contactangle of water on the polymer is greater than the contact angle of wateron the sheet material, then the polymer may be considered a hydrophobicmaterial. If the contact angle of water on the polymer is less than thecontact angle of water on the sheet material, the polymer may beconsidered as a hydrophilic polymer.

Suitable fluorocarbon polymers include fluorine-containing polymers,made by polymerizing or copolymerizing one or more monomers that containat least one fluorine atom. Non-limiting examples of fluorine-containingmonomers that may be polymerized to yield suitable fluorocarbon polymersinclude tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride,perfluoromethyl vinyl ether, perfluoropropyl vinyl ether, and the like.The presence of fluorine-carbon bonds is believed to be responsible forthe hydrophobic nature of these polymers.

A preferred fluorocarbon polymer is polytetrafluoroethylene (PTFE). PTFEis preferred in some embodiments because of its wide availability andrelatively low cost. Other fluorine-containing polymers may also beused. Suitable fluorocarbon polymers include without limitation PTFE;FEP (copolymers of hexafluoropropylene and tetrafluoroethylene); PFA(copolymers of tetrafluoroethylene and perfluoropropylvinylether); MFA(copolymers of tetrafluoroethylene and perfluoromethylvinylether); PCTFE(homopolymers of chlorotrifluoroethylene); PVDF (homopolymers ofvinylidene fluoride); PVF (polymers of vinylfluoride); ETFE (copolymersof ethylene and tetrafluoroethylene); and THV (copolymers of vinylidenefluoride, hexafluoropropylene, and tetrafluoroethylene). Aqueousdispersions of these and other fluorocarbons are commercially available,for example from DuPont. The dispersions may be conveniently prepared byemulsion polymerization of fluorine-containing monomers and othermonomers to form the polymers. Alternatively, the dispersions may bemade by combining polymer powder, solvent, and surfactants. The polymerdispersion may comprise from 1-90% by weight of the fluorocarbon polymerwith the balance comprising water and surfactant. For example, DuPontT30 PTFE product is available containing 60% by weight PTFE particles.

In a non-limiting procedure, the fluorocarbon polymers are applied tothe porous electroconductive substrate by wetting the substrate in awetting composition including the polymer and a liquid. The liquid isalso referred to as a “solvent”. In some embodiments, the wettingcomposition is provided in the form of an emulsion. Solutions may alsobe used. In some embodiments, the wetting compositions containsurface-active materials or other agents to hold the polymer in solutionor suspension, and/or to aid in wetting the substrate. For example, anemulsion used to wet the sheet material may include from 1 to about 70%by weight particles of a hydrophobic polymer such aspolytetrafluoroethylene. In various embodiments, ranges of 1% to 20% arepreferred. In a preferred embodiment, the polymer composition containsapproximately 2% to 15% of the polymer solids by weight.

The liquid is preferably aqueous (water or water mixture), and mayfurther comprise organic liquids. Generally, non-ionic surfactants areused as wetting agents, with the result that no metal ions will be leftin the carbon fiber diffusion media after the wetting agents aredecomposed during high temperature treatment. Non-limiting examples ofsurfactants include nonylphenol ethoxylates, such as the Triton seriesof Rohm and Haas, and perfluorosurfactants.

In a preferred embodiment, a substrate is prepared by applying thefluorocarbon polymer composition to at least one surface of thesubstrate. The polymer composition may be applied to both sides of thesubstrate by immersing the porous substrate (e.g., a carbon fiber paperor cloth) into a fluorocarbon dispersion, by spraying both sides of thesheet-like substrate, or by other suitable means. In a typicalprocedure, the substrate is dipped into the fluorocarbon dispersion andremoved after a time of soak. In other embodiments, the polymercomposition may be applied to only one surface of the substrate, forexample, by spraying, vapor deposition, and the like. Exposure of thesubstrate to the fluorocarbon polymer dispersion occurs for a timesufficient to provide the substrate with the proper amount offluoropolymer. A wide range of loadings of PTFE or other fluorocarbonmay be applied to the carbon fiber substrate. In some embodiments, it isdesirable to incorporate about 2 to 30% fluorocarbon polymer by weightof the diffusion medium, measured after the drying and other steps notedbelow. In other embodiments, at least 5% by weight polymer isincorporated into the diffusion medium. Typically the substrates may bedipped or immersed in the fluorocarbon dispersion for a few minutes toobtain an appropriate loading of fluorocarbon on the substrate. Invarious embodiments, the dispersion contains from 1% to 50% by weight offluorocarbon particles. Dispersions having concentrations of particlesin the preferred range may be made by diluting commercial sources of thedispersions as necessary to achieve the desired concentrations. In anon-limiting example, a dispersion containing 60 weight percent (%) PTFEmay be diluted 20 times with de-ionized water to produce a dispersioncontaining 3% by weight PTFE particles.

The time of exposing the substrate to the fluorocarbon polymerdispersion is long enough for resin particles to imbibe into the poresof the carbon fiber paper or cloth, yet short enough to be aneconomically viable process. Generally, the time of soaking and theconcentration of the fluorocarbon polymer particles, as well as thenature of the resin, may be varied and optimized to achieve desiredresults.

After applying the fluorocarbon polymer composition to at least onesurface of the substrate, it is preferred to remove excess solutionbefore further processing. In one embodiment, the substrate may beremoved from the liquid dispersion and the excess solution allowed todrip off. Other processes are possible, such as rolling, shaking, andother physical operations to remove excess solution.

The diffusion medium is preferably then dried by removing the solvent.Removal of the solvent may be achieved by a variety of methods, such asconvective heat drying or infrared drying.

In addition to the hydrophobic fluorocarbon polymer coating discussedabove, the invention provides in various embodiments for application ofa further surface layer or layers. The most common is referred to as amicroporous layer, conductive particles mixed with a polymeric binder.In various embodiments, the microporous layer is applied as a paste tothe substrate, and may be applied before or after the hydrophobicfluorocarbon polymer coating, or it may be applied to a surface or sideof the medium not covered by the fluorocarbon polymer. As noted, thepaste contains conductive particles and preferably particles of apolymeric binder. Non-limiting examples of conductive particles includecarbon particles such as, without limitation, carbon black, graphiteparticles, ground carbon fibers, and acetylene black. The polymericbinder is preferably made of a fluorocarbon polymer or fluororesin suchas discussed above with respect to the first coating on the substrate.In this regard a preferred fluorocarbon polymer for making the paste isPTFE. In various embodiments, the paste is applied to the substrate toform the microporous layer by conventional techniques, such as doctorblading, screen printing, spraying, and rod coating.

In practice, the paste is made from a major amount of solvents and arelatively lesser amount of solids. The viscosity of the paste can bevaried by adjusting the level of solids. The solids contain both thecarbon particles and the fluorocarbon polymer particles in a ratio byweight of from about 9:1 to about 1:9. Preferably, the weight ratio ofcarbon particles to fluorocarbon polymer is from about 3:1 to about 1:3.The fluorocarbon particles are conveniently supplied as a dispersion inwater. An exemplary paste composition contains 2.4 grams acetyleneblack, 31.5 mL isopropanol, 37 mL deionized water, and 1.33 g of a 60%by weight dispersion of PTFE in water. This paste has a weight ratio ofacetylene black to fluorocarbon polymer, on a dry basis, of about 3:1.

The paste is applied onto the dried porous substrate to provide amicroporous layer that extends from the surface into the interior of thepaper. In various embodiments, the microporous layer is about 5 to about20% of the thickness of the paper. For example, with a typical paper 200microns thick, the microporous layer is from about 10 to about 30microns thick above the surface of the paper. Penetration of themicroporous layer into the bulk of the paper can range up to about 100μm, and depends on the viscosity of the paste. The amount of paste toapply to a paper can be determined from the density of the solids, thearea of the paper, and the thickness of microporous layer desired. Invarious embodiments, a paste is applied to a paper at an areal loadingof about 1.0 to about 2.5 mg/cm², based on the weight of the solids inthe paste.

The microporous layer preferably has a pore size of the carbonagglomerates, i.e., between about 100 and about 500 nm, as compared with10 to 30 microns pore size for carbon fiber paper substrates. Themicroporous layer provides effective removal of liquid water from thecathode catalyst layer into the diffusion media. For this reason, thediffusion medium is preferably installed in the fuel cells with themicroporous layer side toward the cathode. The microporous layer alsoaides in reducing electrical contact resistance with the adjacentcatalyst layer. The properties of the microporous layer can be varied byadjusting the hydrophobicity of the polymeric binder and the particleand agglomerate structure of the conductive particles.

After optional application of the microporous layer, the substrate ispreferably sintered by heating at a temperature high enough to melt theparticles of polymeric binder and coalesce the microporous layer. In thecase of PTFE containing microporous layers, a temperature of 380° C. hasbeen found to sufficient.

After the first coating that comprises a hydrophobic polymer is appliedand adhered to the substrate, a second coating comprising a hydrophobicsilicone polymer is adhered to the substrate. Any first coating and/ormicroporous layer is to be sintered prior to applying the second(silicone) coating.

The second coating results in a hydrophobic silicone layer being appliedto the substrate. A hydrophobic silicone layer is one that gives acontact angle of water of greater than 90°. Although the invention isnot limited by theory, it is believed that the second coating and theresulting hydrophobic silicone coating sticks or adheres to portions ofthe substrate that are not adequately covered by PTFE or otherfluorocarbon polymers. Without the silicone coating, these areas of thesubstrate would remain more hydrophilic than the other coated areas andbecome more hydrophilic over time during fuel cell operation. It isbelieved that such hydrophilic areas, which result from incompletecoverage of the substrate by the fluorocarbon polymer, lead to poorperformance, especially at high humidity, of cells that contain suchcoated substrates as diffusion media. One reason for this may be thatthe hydrophilic areas on the incompletely covered substrate tend toretain water rather than repel water. In addition, heterogeneous surfaceproperties of the gas diffusion media facing the flow channels makes itmore difficult to remove water slugs in the gas flow channels, which canresult in uneven gas flow distributions among different cells. As aresult, water tends to accumulate and inhibit the electrochemicalreactions of certain cells, which results in so-called low performingcells. By covering any such small hydrophilic areas on the fluorocarboncoated substrate with additional hydrophobic polymer, such as the secondhydrophobic silicone polymer coating, the diffusion medium is renderedmore homogeneously hydrophobic. As a result, the performance of fuelcells containing the substrates, especially at high relative humidity,is improved.

In various embodiments, the second coating is adhered to the substrateby contacting the diffusion medium with a silicone composition thatcontains a curable silicone resin, preferably one that is curable by theaction of moisture. In various embodiments, the silicone composition isapplied to the substrate by dipping, spraying, or other means.Conveniently, the silicon composition contains a solvent in addition tothe silicone resin components. If the silicone resin is moisturecurable, it is preferred to use a solvent other than water. Preferablythe solvent is one that does not interfere with the cure of the siliconeor with the application of the silicone coating onto the substrate.Methylene dichloride has been found to be a suitable solvent.

In a preferred embodiment, the curable silicone resin is one in whichthe cure is activated by contact with moisture. In various embodiments,the curable silicone resin contains a silicone prepolymer and acrosslinker. The prepolymer and the crosslinker contain functionalgroups that react with one another preferably activated by the presenceof water or moisture, to form a crosslinked or cured silicone polymer.The resulting polymer, formed on the surface of the substrate, andpreferably on areas of the substrate not adequately coated by thefluorocarbon polymer, is hydrophobic and acts to repel water and preventaccumulation on the cathode of the fuel cell. As a result, performanceof the fuel cell, especially at high relative humidity, is enhanced.

Advantageously, after application of a moisture curable silicone resinas described above, curing occurs upon exposure to moisture in air, andcan further continue after assembly into the fuel cell and fuel cellstack during operation of the fuel cell stack that produces water.

Illustrative silicone prepolymers include those that are represented bythe following structure

In the structure R¹ and R² are the same or different and independentlyrepresent aliphatic or aromatic groups. In various embodiments, thealiphatic or aromatic groups R¹ and R² comprise alkyl, aryl,perfluoroalkyl, and perfluoroaryl, preferably having from 1 to 20carbons. In an embodiment, R¹ and R² independently contain from 1 to 6carbons. The aliphatic and aromatic groups R¹ and R² can contain etherlinkages, as long as the resulting polymer upon cure is hydrophobicenough to enhance performance of a fuel cell containing a coateddiffusion medium. For example, the R¹ and R² groups can independentlycomprise polyethers such as polypropylene oxides and polyperfluoroolefinethers. The groups X¹ and X² are chemical moieties that provide thesilicone prepolymer with the ability to react with complementaryfunctional groups on the crosslinker to form the cured hydrophobicpolymer on the surface of the substrate. Preferred X¹ and X² groupsinclude hydrogen and lower alkyl having 1 to 6 carbons. The siliconeprepolymer has a molecular weight determined by the value of n, whichrepresents the average number of repeating siloxane units in theprepolymer. Generally, n is greater than 2 and is typically about200-1500 for room temperature vulcanized materials and 3000-11000 forheat cured materials. Typical values of n range from about 10 to about1000.

In various embodiments, the crosslinker has a structure defined by thefollowingSiY_(m)R³ _(4-m)   (2)where the R³ are independently a group that does not participate in thecrosslinking. Non-limiting examples include aliphatic and aromaticgroups such as discussed above for the prepolymer. In preferredembodiments, R³ is selected from among C₁₋₆ alkyl, and C₆₋₁₀ arylgroups. Preferred R³ groups include methyl and hydrogen. Y is afunctional group that is reactive with the X¹O and X²O groups of theprepolymer to form covalent bonds to crosslink and cure the resin.Preferred functional groups for element to Y includes hydroxyl andalkoxy, especially alkoxy of 1 to 6 carbon atoms. In a preferredembodiment, the Y group is methoxy or ethoxy. In the crosslinker, m isgreater than 1 and is preferably 2 or greater. In a preferredembodiment, m is greater than 2 and preferably less than 4.

Preferably, the groups X¹ and X² and Y are such that crosslinking isenhanced by the action of water. For example, when X¹ and X² arehydrogen or alkoxy and Y is hydroxyl or alkoxy, reaction of water actsto enhance the crosslinking reaction.

The silicone resins optionally further comprise suitable additives suchas fillers, other auxiliaries, chain transfer agents for controllingpolymerization, and the like. Suitable silicone resins are commerciallyavailable. Non-limiting examples include Dow Corning 3140 adhesive. Thehydrophobic silicone coating is applied by contacting the substrate witha silicone composition as described above. A variety of methods can beused to carry out the contact such as dipping, spraying, rolling, doctorblading, and the like. In a non-limiting example, the substrate isdipped into a dilute solution of the curable silicone resin. Thedilution of the resin and the method of application are chosen so as toapply sufficient silicone polymer to make the resulting diffusion mediumhydrophobic and improve cell performance, especially at high relativehumidity. It has been found acceptable to add about 1% of siliconepolymer, based on the total weight of the diffusion medium.

In various embodiments, the methods described above can be used to carryout a kind of repair on a fuel cell stack where certain cells in thestack are exhibiting low performance at high relative humidity operationconditions and/or low current operation conditions (e.g. much lower gasflow rate and thus hard to remove water slugs in the gas flow channels).As discussed, such fuel cell stacks normally contain a large number ofindividual fuel cells depending on the power required. Typically fuelcell stacks contain 20 to 500 cells, 50 to 500 cells, 100 to 400 cells,or 200 to 400 cells. Because the cells are installed in series, any lowperforming individual cell will affect the performance of the fuelstack. In one embodiment, the methods above can be used to repair orremediate an individual fuel cell operating in such a fuel stack. Inthis method, the low performing cell in the stack is identified and thediffusion medium of the cell is treated by contacting it with a curablesilicone resin. Upon reassembly of the fuel cell and the fuel cellstack, it is observed that the high humidity performance of the cell andlow current stability is improved.

Instead of, or in addition to fuel cell remediation as discussed above,the methods of the invention can be applied prophylactically to makehydrophobic diffusion media that work more reliably, especially whencombined into fuel stacks containing 50, 100, 200, or more individualfuel cells.

Diffusion media prepared by coating with fluorocarbon polymers tend alsoto gradually lose hydrophobicity over time during stack operation. Forexample, over time, dark lines can be observed forming on such carbonfiber diffusion media. The dark lines represent areas less hydrophobicthan the freshly coated sample. Accordingly, water tends to accumulateat those areas of the diffusion medium. In various embodiments, treatingfluorocarbon polymer coated diffusion media with hydrophobic siliconeresin as described herein mitigates this loss of hydrophobicity overtime of stack operation. Such resistance to aging can be observed, forexample in laboratory ex situ aging tests. In such aging tests, coateddiffusion media are exposed to 15% hydrogen peroxide solution at 65° C.for extended periods of time, for example 7 or 10 days. Wilhelmymeasurements of advancing and receding contact angles indicate responseof the substrate to aging. In the case of substrate dipped in siliconeresin to apply the second hydrophobic silicone coating, the contactangles tend to remain constant or even go up slightly upon aging,reflecting good retention of hydrophobicity upon accelerated aging.

The invention described above with respect to various embodiments.Further non-limiting descriptions are given in the working Examples thatfollow.

EXAMPLES Example 1 Preparation of Fluorocarbon Polymer Coated DiffusionMedium

A fluoropolymer solution is prepared by diluting one part DuPont T30solution into 19 parts deionized water by volume. T30 is a commercialproduct and is a dispersion of 60% by weight PTFE, water, andsurfactant. The diluted solution is stirred for about 5 minutes. Toraycarbon paper (TGPH 060 or TGPH 090) is immersed into the fluoropolymersolution for a time period of about 3 minutes up to about 5 minutes. Thepaper is then removed form the solution and allowed to drip dry forabout 1 minute. After drip dry, the excess solvent is removed from thepaper by placing the paper in a 90° C. oven until dry. The temperatureis then ramped up to 290° C. and then 380° C. in the oven and held atthe respective temperatures for 30 minutes. The resulting diffusionmedium contains a hydrophobic fluorocarbon polymer adhered to thesubstrate. The polymer coating on the substrate is sintered.

A number of carbon fiber substrates are fluorocarbon coated as describedin the preceding paragraphs. Although most of the papers prepared insuch a fashion exhibit suitable performance at high relative humidity,an individual coated substrate having low performance is identified andfurther tested below.

Example 2

The gas diffusion medium with low performance (poor water managementcapability) identified in Example 1 is treated with a silicone coatingby dipping it into a dichloromethane solution containing 2 giliter DowCorning 3140 adhesive. The substrate is then air dried and installedinto a fuel cell.

Example 3

A low performing diffusion medium identified from Example 1 and adiffusion medium further treated with silicone as described in Example 2are tested in fuel cells, with the results shown in FIG. 2. The figuresshow current versus voltage curves. The fuel cell is composed of a pairof serpentine graphite flow fields with 50 cm² active area. The MEA usedin the test is Gore 5510 (25 microns thick) MEA. Three differentoperating conditions are tested: 1) cell temperature 70° C., anode ispure H₂, cathode is air, the gas outlet is pressure 100 KPa (absolute),and the inlet gas for the anode and cathode are under 40% relativehumidity that results in 84% outlet relative humidity during operation;2) cell temperature 80° C., anode is pure H₂, cathode is air, the gasoutlet is pressure 150 KPa (absolute), and the inlet gas for the anodeand cathode are under 66% relative humidity that results in 110% outletrelative humidity during operation; and 3) cell temperature 60° C.,anode is pure H₂, cathode is air, the gas outlet is pressure 270 KPa(absolute), and the inlet gas for the anode and cathode are under 100%relative humidity that results in 300% outlet relative humidity duringoperation. At 84% outlet relative humidity, the current versus voltagecurves for the diffusion media of Example 1 and Example 2 are nearlyidentical.

FIGS. 2 a and 2 b show, respectively, the voltage curves for comparativefuel cells run at 110% outlet relative humidity and 300% outlet relativehumidity, respectively. The Figures show that the cell performance offuel cells containing silicone coated diffusion media (Example 2) issuperior to those having only the fluorocarbon polymer coating (Example1).

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A diffusion medium for use in a PEM fuel cell, comprising: a porousconductive substrate; a first coating comprising a hydrophobicfluorocarbon polymer adhered to the substrate; and a second coatingcomprising a silicone polymer adhered to the substrate.
 2. A diffusionmedium according to claim 1, wherein the substrate comprises carbonfiber paper.
 3. A diffusion medium according to claim 1, wherein thehydrophobic fluorocarbon polymer comprises PTFE.
 4. A diffusion mediumaccording to claim 1, wherein the first coating adheres to the substrateover a major part of the surface area of the substrate and the secondcoating adheres to an area of the substrate not covered by the firstcoating.
 5. A diffusion medium according to claim 1, wherein the secondcoating comprises a moisture cured silicone.
 6. A diffusion mediumaccording to claim 1, wherein the diffusion medium has a first andsecond side, the first side comprising the first and second coating, thesecond side comprising a microporous layer, wherein the microporouslayer comprises carbon and a sintered mass of fluorocarbon polymerparticles.
 7. A fuel stack comprising a plurality of fuel cells, whereinat least one of the fuel cells comprises a diffusion medium according toclaim
 1. 8. A fuel stack comprising a plurality of fuel cells, whereinat least one of the fuel cells comprises a diffusion medium according toclaim
 6. 9. A method for making a diffusion medium for use in a PEM fuelcell, comprising: coating a porous conductive substrate with ahydrophobic fluorocarbon polymer, and contacting the fluorocarbon coatedsubstrate with a silicone composition comprising a moisture curablesilicone resin.
 10. A method according to claim 9, wherein coatingcomprises contacting the substrate with a coating composition comprisinga solvent and particles of a hydrophobic fluorocarbon polymer, removingthe solvent, and sintering the particles.
 11. A method according toclaim 10, comprising loading the substrate with 1 to 20% by weight ofthe fluorocarbon particles, based on the total weight of the coatedsubstrate after sintering.
 12. A method according to claim 9, comprisingloading the substrate with 3 to 10% by weight of the fluorocarbonparticles, based on the total weight of the coated substrate aftersintering.
 13. A method according to claim 9, wherein contactingcomprising spraying the silicone composition onto the substrate.
 14. Amethod according to claim 9, further comprising applying a microporouslayer to one side of the substrate, wherein the microporous layercomprises fluorocarbon polymer and conductive particles.
 15. A methodfor improving the high humidity performance of a PEM fuel cell stack,the stack comprising a plurality of PEM fuel cells, the fuel cellscomprising a cathode, an anode, a polyelectrolyte membrane disposedbetween the cathode and the anode, flow fields adjacent the electrodes,and a fluoropolymer-coated diffusion medium disposed between at leastone of the cathode and the cathode flow field and the anode and theanode flow field, the method comprising contacting the fluoropolymercarbon diffusion medium with a silicone composition comprising amoisture curable silicone resin.
 16. A method according to claim 15,wherein the diffusion medium is disposed between the cathode and thecathode flow field.
 17. A method according to claim 15, comprisingproviding the diffusion medium with a hydrophobic coating on areas ofthe diffusion medium not covered by fluorocarbon polymer.
 18. A methodaccording to claim 15, comprising contacting the diffusion medium withthe silicone composition prior to assembly of the stack.
 19. A methodaccording to claim 15, comprising identifying a low-performing cellduring operation of the stack, contacting the diffusion medium of thelow-performing cell with the silicone composition, and continuingoperation of the stack.
 20. A method according to claim 19, comprisingremoving the diffusion medium of the low-performing cell from the stack,treating the diffusion medium by contacting it with the siliconecomposition, and reassembling the stack with the treated diffusionmedium.